In a radio communication system, since the propagation environment varies every instant, it is necessary for a signal receiving side to compensate a received signal for the influence of the propagation environment. Therefore, the signal transmitted in the radio communication system generally contains a known pilot symbol. The signal receiving side detects the state of distortion of the pilot symbol by channel estimation, and, using the result, compensates data symbols including information for the influence of the propagation environment.
Specifically, for example, as shown in FIG. 1, the signal transmitting side places a pilot symbol (diagonally shaded areas in the figure) at the beginning of a frame, and data symbols (white areas in the figure) subsequent to the pilot symbol. Then, the receiving side performs channel estimation using the pilot symbols of two consecutive frames, performs interpolation, for example, and thereby compensates the data symbols over these two pilot symbols for the propagation path variation.
Data symbols are thus compensated for the propagation path variation based on channel estimation results of the pilot symbols arranged to sandwich the data symbols. Therefore, when the interval between pilot symbols is decreased, the accuracy (the propagation path compensation of the data symbol) improves. In other words, when the proportion of pilot symbols (in a frame) is increased, data symbols are received with higher accuracy.
However, since the pilot symbol does not include information to be transmitted, when the proportion of pilot symbols (in a frame) is increased, the proportion of data symbols is decreased and the information transmission efficiency is reduced.
In view of the foregoing, for example, Patent Document 1 discloses a technique for adaptively determining the subcarrier in which a pilot symbol is inserted according reception power differences between subcarriers with different frequencies in OFDM (Orthogonal Frequency Division Multiplexing). In the technique disclosed in Patent Document 1, (the signal receiving side) determines a subcarrier to insert a pilot symbol, and transmits information on the subcarrier to a signal transmitting side as feedback. Then, according to this feedback information, the signal transmitting side inserts the pilot symbol to transmit.
Patent Document 1: JP 2003-174426